1. Field of the Invention
The present invention relates to nano-imprint lithography, and more particularly to a process and an apparatus for ultraviolet nano-imprint lithography, which accomplishes an accurate layer-layer alignment and an easy removal of residual layers after imprint processes.
2. Description of the Related Art
Photolithography techniques have been used to make most microelectronic devices. However, it is believed that these methods are reaching their limits in resolution. Sub-micron scale lithography has been a critical process in the microelectronics industry. The use of sub-micron scale lithography allows manufacturers to meet the increased demand for smaller and more densely packed electronic components on chips.
There are emerging applications of nanometer scale lithography in the areas of optical electronics and magnetic storage. For example, photonic crystals and high-density patterned magnetic memory of the order of terabytes per square inch require nanometer scale lithography.
For making sub-50 nm structures, photolithography techniques may require the use of very short wavelengths of light (e.g., about 13.2 nm). At these short wavelengths, many common materials may not be optically transparent and therefore imaging systems typically have to be constructed using complicated reflective optics. Furthermore, obtaining a light source that has sufficient output intensity at these wavelengths may be difficult. Such systems may lead to extremely complicated equipment and processes that may be prohibitively expensive. It is believed that high-resolution e-beam lithography techniques, though very precise, may be too slow for high-volume commercial applications.
Since around 1995, a nano-imprint lithography (NIL) technology has been noted as an efficient and economical pattern forming technology. In the nano-imprint lithography, a SiO2 concave-convex pattern (a patterned template or a mold) formed on silicon (Si) is press-transferred to a resist.
By means of this technology, it has become possible to form an ultra-micro pattern on a larger area.
Nano-imprint lithography processes have demonstrated the ability to replicate high-resolution (sub-50 nm) images on substrates using templates that contain images as topography on their surfaces. It is believed that nano-imprint lithography may be an alternative to optical lithography for use in patterning substrates in the manufacture of microelectronic devices, such as MOSFET, MSM, a diffraction grid, optical electronic devices, patterned magnetic media for storage applications. Nano-Imprint lithography techniques may be superior to photolithography, because the former enables a transfer of a total pattern, while the latter does not.
In the nano-imprinting technology, a mold pattern is directly transferred to a substrate using a press. Further, unlike the photolithography process which requires several steps to produce a desired substrate, the nano-imprinting technology, in which a pattern is directly transferred to a substrate by a relatively small number of steps. Thus, this technology appears to be very effective lithography, especially when forming a multi-structure.
In the earliest nano-imprit lithography process, a thin layer of imprint resist (thermoplastic polymer) is spin coated onto a sample substrate. Then the mold, which has predefined topological patterns, is brought into contact with the sample and they are pressed together under certain pressure. When heated up above the glass transition temperature of the polymer, the pattern on the mold is pressed into the melt polymer film. After being cooled down, the mold is separated from the sample and the pattern resist is left on the substrate. A pattern transfer process (Reactive Ion Etching, normally) can be used to transfer the pattern in the resist to the underneath substrate. This process is named Hot Embossing or Thermal Imprint Lithography.
However, the thermal imprint lithography employs high pressure and heat, which require complicate and expensive equipment, causing increase in the cost of manufacture. Also, the thermal imprint lithography is difficult to perform on a large substrate and the substrate is may be distorted or damaged due to the high pressure.
As an alternative to the thermal imprint lithography, an UV assisted Imprint Lithography has been proposed. In the UV assisted Nanoimprint Lithography, a photo (UV) curable liquid resist is applied to the sample substrate and the mold is normally made of transparent material. After the mold and the substrate are pressed together, the resist is cured in UV light and becomes solid. After mold separation, a similar pattern transfer process can be used to transfer the pattern in resist onto the underneath material. This method, which was proposed by C. G. Willson and S. V. Sreenivasan of Texas University, can be carried out a pressure as low as below the normal pressure.
Further, since the curing is performed by the UV treatment, a transparent mold made of quartz material should be used. This method includes a number of advantages over the conventional imprint method, but it is considerably difficult to engrave the nano-size pattern on the quartz mold, and thus the cost for manufacturing the mold is very high. Also, when contacting the curable liquid resist to the mold, air bubbles are trapped to cause defects of the products. Control of such defects can increase costs. Currently, the two imprint methods are dividedly used in the fields of the art, and applied to different technical fields depending on their advantages and disadvantages.
Recently, three topics are emerging with respect to the NIL operation: a first topic is related to a larger area process; a second one is an effective removal of residual layers; and a third one is an alignment of layers. Additionally, there are many efforts to cleanly separate the mold from the substrate and to lower the pressure and the temperature used.
First, with respect to a larger area process, it has been frequently reported that, the process for 4-inch area could be successfully performed without the defects. However, with respect to a wafer larger than that size, there has not been reported that the pattern transfer to such a wafer was completely performed in a single process. It is because the application of the uniform pressure to such a larger area is difficult and because the process is not economic owing to the expensive mold.
Accordingly, in the recent days, the step-and-repeat method is being widely used, in which an imprint field (die) which is typically much smaller than the full wafer nanoimprint field, is repeatedly imprinted to the substrate with certain step size. In such a method, a wafer having an area over 8 inches in diameter can be theoretically subjected to the process without causing troubles.
Second, the residual layer following imprint appears a universal problem in the molding process. To remove the residual layers, an etching process is needed. This seems to be aggravated particularly in a region with less than 100 nanometers in size. Accordingly, there are needs to effectively remove residual layers.
Third topic is the alignment of layers. In the photolithography processes, the light-exposure process is performed in a non-contact manner, the alignment of layers is not a big issue Therefore, the photolithography process is advantageous when applied to a curved surface or a multi-layered structure. The nano-imprint lithography process, on the other hand, when applied to a curved surface or a multi-layered structure, may not be advantageous because there is higher probability of failing to transfer pattern to the substrate and an accurate alignment of contact (or very close) layers. Use of an alignment marker has been proposed to improve the alignment of layers. The precision currently obtained using such an align marker is a degree of a few hundreds nanometer and there still are needs to improve the alignment.